1. Field of Invention
The present invention relates to a projection-type display apparatus that modulates an output light beam from a light source in response to a video signal using a modulating device such as a liquid-crystal light valve or the like and enlarges and projects the modulated light beam through a projection lens onto a screen. More particularly, the present invention relates to a projection-type display apparatus that illuminates an image forming area of the modulating device such as a liquid-crystal light valve.
2. Description of Related Art
Japanese Unexamined Patent Publication 3-111806 discloses a projection-type display apparatus that forms a modulated light beam in response to a video signal using a liquid-crystal light valve and enlarges and projects the modulated light beam onto a screen. As shown in FIG. 13, the projection-type display apparatus includes an integrator optical system 923 having two lens plates 921 and 922, for uniformly illuminating the image forming area of a liquid-crystal light valve 925, as a modulating device, with light from a light source.
The single light beam emitted from a light source lamp unit 8 is split into a plurality of intermediate light beams by lenses 921a constituting the first lens plate 921, and is superimposed on the liquid-crystal light valve 925 through lenses 922a constituting the second lens plate 922.
In this projection-type display apparatus, the brightness of an image projected onto the screen will drop or the projected image suffers an outlining shadow if the image forming area of the liquid-crystal light valve 925 is not accurately illuminated. To cope with this problem, as shown in FIG. 14, a certain margin M is assured around the image forming area A of the liquid-crystal light valve 925 in view of the positioning accuracies of the liquid-crystal light valve 925 and the lens plates 921, 922 constituting the integrator optical system 923, the positional errors in the focal points of the lenses 921a, 922a of the lens plates and the positioning accuracies of the remaining optical systems disposed along the optical axis.
More particularly, the image forming area A is sized to be distinctly smaller than an illumination area B by the output light from the light source. Even if the illumination area B is offset vertically upward or downward, or horizontally leftward or rightward depending on the overall positioning accuracy of the above components, the image forming area A is designed to be kept within the illumination area B. This avoids the outlining shadow around the projected image and drop in brightness of the projected image. To cope with a large quantity of positional error of the components, the margin M is simply widened.
To increase the brightness of a projected image, the efficiency in the utilization of the light that illuminates the liquid-crystal light valve 925 needs to be increased. If the margin M is enlarged to accommodate errors of the components, the efficiency in the utilization of light drops, thereby dimming the projected image. Thus, the width of the margin formed around the image forming area of the liquid-crystal light valve is preferably narrow. A narrow margin in turn offsets the illumination area from the image forming area of the liquid-crystal light valve, possibly presenting a shadow outlining the projected image.